1. Field of the Invention
The present invention relates to a swash-plate compression device of the variable capacity type used for indoor air conditioning, and more particularly, to a compression device suited for use in an air conditioning system of a vehicle.
2. Description of the Related Art
A swash-plate compression device of this type is described in Jpn. Pat. Appln. KOKAI Publication No. 2001-107854, for example. This conventional compression device comprises a swash-plate compressor, which includes a crank chamber, a swash plate rotatable in the crank chamber, and a plurality of pistons that reciprocate as the swash plate rotates. The reciprocation stroke of each piston settles the capacity of the compressor.
On the other hand, the tilt angle of the swash plate, which settles the reciprocation stroke of each piston, is adjusted by means of the pressure in the crank chamber. Accordingly, the capacity of the compressor of this type can be varied by adjusting the pressure in the crank chamber.
If the compression device described above is used in an air conditioning system, the compressor is inserted in an external refrigerant circulation path. A refrigerant that is compressed by the compressor at high pressure is discharged from the compressor into the circulation path. In a compression device, in general, the pressure in its crank chamber can be autonomously adjusted. More specifically, the pressure in the crank chamber is feedback-controlled so that an actual differential pressure of the refrigerant between two given points in the external circulation path is equal to a target differential pressure. In consequence, the capacity of the compressor is varied by the pressure control in the crank chamber. More specifically, the actual differential pressure of the refrigerant is obtained from refrigerant pressures at two points between the compressor and a condenser, while the target differential pressure is settled according to external information from various external information detectors.
As mentioned before, the compression device autonomously adjusts the pressure in the crank chamber. Therefore, the compression device described in the aforesaid publication further comprises a refrigerant passage, through which some of the refrigerant discharged from the compressor is introduced into the crank chamber, and a solenoid valve inserted in the refrigerant passage.
The solenoid valve includes a valve body that adjusts the opening of the refrigerant passage. An electromagnetic force corresponding to the target differential pressure is applied to the valve body. On the other hand, the valve body is subjected to the aforesaid actual differential pressure of the refrigerant in the direction opposite to direction of the electromagnetic force. Thus, the refrigerant passage or the opening of the solenoid valve is adjusted in accordance with the electromagnetic force and the actual differential pressure. In consequence, the feed of the refrigerant into the crank chamber is adjusted, so that the pressure in the crank chamber (or the capacity of the compressor) can be feedback-controlled in accordance with the target differential pressure.
For the feedback control of the pressure in the crank chamber, the compression device described in the aforesaid publication uses the actual differential pressure of the refrigerant between two points in the external circulation path. In order to stabilize the feedback control, in this case, the actual differential pressure of refrigerant should be increased to a high level. To attain this, the compression device has a throttle that restrains the flow of the refrigerant between the two points in the circulation path. However, this throttle lowers the pressure of the refrigerant that is supplied from the compressor to the condenser, thereby causing a loss of the refrigerant pressure and lowering the efficiency of the air conditioning system.